Fine mold for molding fuel cell separator, method for producing fuel cell separator, and fuel cell separator

ABSTRACT

Provided is a fine mold for molding a fuel cell separator by press-molding a metal thin plate for a fuel cell separator to produce the fuel cell separator, comprising: a molding surface in which a concavity and convexity are adjacent to each other with a prescribed period, and (i) an arc-like minute concave surface on an upper surface of the concavity and convexity, and (ii) an arc-like minute convex surface on a lower surface of the concavity and convexity.

TECHNICAL FIELD

The present invention relates to a fine mold for molding a fuel cellseparator, a method for producing a fuel cell separator using the mold,and a fuel cell separator.

BACKGROUND ART

These days, polymer electrolyte fuel cells are used for automobilesusing electric power as the driving source, small-scale electricitygenerating systems, and the like. The basic components of the polymerelectrolyte fuel cell are electrodes and separators having passages in afine concave-convex shape that supply fuel gas (reaction gas). Theseparator is produced by press-molding a metal thin plate using a fineconcave-convex mold.

In order to reduce the contact resistance and make it easy for reactiongas (hydrogen and air) to flow, the cross section of the passages of theseparator is preferably an acute angle-like rectangular cross section inwhich the top of the convexity is flat and the standing wall sharedbetween the concavity and convexity is vertical.

As the metal thin plate for a separator, a titanium thin plate or astainless steel thin plate is usually used from the viewpoints ofcorrosion resistance and durability; however, it is technicallydifficult to form concavities and convexities (passages) having thepreferred rectangular cross section mentioned above on the metal thinplate using an concave-convex mold while preventing a crack associatedwith a decrease in the thickness of the metal thin plate and suppressinga “warp” due to elasticity recovery after the press-molding; thus, sometechnologies of using an concave-convex mold to perform press-processingon a metal thin plate to form concavities and convexities (passages)have been proposed until now.

Patent Literature 1 proposes a method for press-processing a platematerial involving forming a plurality of protrusions on a plasticallydeformable plate material by pressing, the method comprising a firststep of using a first press mold to form a protruding portion in aportion where it is intended to form a protrusion and a second step ofusing a second press mold to push an inside portion that is nearer tothe center of the protruding portion than to the peripheral portion ofthe protruding portion formed in the first step and thereby completing aprotrusion.

In the press-processing method of Patent Literature 1, the warp of theplate material after the protrusion formation can be suppressed; butsince consideration is not given to forming the standing wall of theprotrusion vertically, a protrusion having an acute angle-likerectangular cross section in which the top of the protrusion is flat andthe standing wall of the protrusion is vertical cannot be formed.

Patent Literature 2 proposes a method for producing a separator for apolymer electrolyte fuel cell that has a flat portion at the peripheryand has convexities and concavities serving as gas passages in a portionexcluding the periphery, the method comprising molding a material into across-sectional shape of continuously repeated convexities andconcavities as preliminary molding and then molding the material intothe final cross-sectional shape of repeated convexities and concavities.

However, in the production method of Patent Literature 2, sinceconsideration is not given to making the top of the convexity flat, anacute angle-like rectangular cross section in which the top of theconvexity is flat and the standing wall shared between the concavity andconvexity is vertical cannot be formed.

Patent Literature 3 proposes an apparatus for producing a separator fora polymer electrolyte fuel cell that has a flat portion at the peripheryand has convexities and concavities serving as gas passages in a portionexcluding the periphery, in which the clearance c (mm) of the standingwall portion of the concave-convex portion, the radius r (mm) of theshoulder, the depth d (mm) of the groove, and the period of p (mm) ofthe groove satisfy a prescribed relational expression comprising theplate thickness t (mm) of the material to be processed.

However, even when the production apparatus of Patent Literature 3 isused, an acute angle-like rectangular cross section in which the top ofthe convexity is flat and the standing wall shared between the concavityand convexity is vertical cannot be formed.

Patent Literature 4 proposes a method for producing a metal separatorfor a fuel cell, the method comprising, when a metal separator for afuel cell that has gas passages having a concave-convex cross-sectionalform with a draft angle of less than or equal to 50° and an inner radiusof less than or equal to 0.5 mm is produced by press-molding on amaterial plate of a separator in which electrically conductiveinclusions protrude on the surface, subjecting the material plate to aprimary molding of protrusion-molding the gas passage up to a surfacearea of more than or equal to 80% of the target surface area of the gaspassage, using a mold in which the molding portion of the protrudingportion is in a R shape, and then subjecting the material plate to asecondary molding of press-molding the gas passage into the final shape.

However, the production method of Patent Literature 4 is unclear in thefinal shape of the gas passage, and is not a method of forming a gaspassage having an acute angle-like rectangular cross section in whichthe top of the convexity is flat and the standing wall shared betweenthe concavity and convexity is vertical.

Patent Literature 5 proposes a method for molding passages of metalseparators, which are stacked on a cell of a fuel cell and form fuel gaspassages and oxidizing gas passages for the cell by means of concavitiesand convexities molded by pressing, the method comprising a first stepin which lengthwise concavities and convexities are formed on a flatplate-like metal plate by press-processing, a second step in which theconcave bottom surface of the concavities and convexities formed in thefirst step is pushed from the outside surface of the concave bottomsurface, the convex top surface is pushed from the outside surface ofthe convex top surface, and thus the concave bottom surface and theconvex top surface are formed into a concave curved surface runningalong the longitudinal direction of the concavities and convexities, anda third step in which the concave bottom surface formed in the secondstep is pushed from the inside surface of the concave bottom surface,the curved surface of the convex top surface is pushed from the insidesurface of the convex top surface, and thus the concave bottom surfaceand the convex top surface are formed into a flat surface.

In the molding method of Patent Literature 5, after the preliminarymolding in the first step, a curved surface is formed on the convexityin the second step, and the curved surface is squashed into a flat shapein the third step; however, the squashing leaves a mark on the surfaceand the surface is roughened, and consequently an acute angle-likerectangular cross section in which the top of the convexity is flat andthe standing wall shared between the concavity and convexity is verticalcannot be formed. Furthermore, in the molding method of PatentLiterature 5, since the third step of squashing the curved surface ofthe convexity is needed, productivity is low.

Patent Literature 6 proposes a method for producing a separator for apolymer electrolyte fuel cell, in which a metallic glass plate materialwith a thickness of 0.02 to 0.5 mm is prepared, the metallic glass platematerial is press-processed in a state of being heated in thesupercooled liquid region of the glass transition temperature to thecrystallization temperature to form concavities and convexities servingas gas passages, and subsequently a film of an oxide and/or a nitride isformed on the surface on which concavities and convexities have beenformed.

The production method of Patent Literature 6 is a method of producing ametallic glass separator, and in Patent Literature 6 only the concaveshape of the top is illustrated. Therefore, in the production method ofPatent Literature 6, an acute angle-like rectangular cross section inwhich the top of the convexity is flat and the standing wall sharedbetween the concavity and convexity is vertical cannot be formed.

In Patent Literature 7, there is a description concerning a separatorproduction method in which a separator material is molded to be providedwith concavities and convexities to obtain a wavelike cross-sectionalform and is then subjected to coining partly, and thereby the occurrenceof a crack, a distortion, and a warp is suppressed.

In the production method of Patent Literature 7, the coining process isperformed after the molding of obtaining a wavelike cross-sectionalform; if it is attempted to perform coining in the first step, theinflow of the material in the cross section stops, and consequently thetension becomes too large and a crack occurs. Hence, a plurality ofsteps are needed, and production costs are increased. If coining is usedfor an acute angle-like rectangular cross section in which the angle ofthe standing wall is vertical or nearly vertical, the tension of thestanding wall becomes large during coining molding, and a crack mayoccur.

CITATION LIST Patent Literature

Patent Literature 1: JP 2000-317531A

Patent Literature 2: JP 2002-313354A

Patent Literature 3: JP 2004-265856A

Patent Literature 4: JP 2005-243252A

Patent Literature 5: JP 2006-120497A

Patent Literature 6: JP 2007-066817A

Patent Literature 7: JP 2007-48616A

SUMMARY OF INVENTION Technical Problem

As described above, the cross section of the passages of a separator ispreferably an acute angle-like rectangular cross section in which thetop of the convexity is flat and the standing wall shared between theconcavity and convexity is vertical in order to reduce the contactresistance between the separator and the polymer electrolyte membraneand to make it easy for reaction gas (hydrogen and air) to flow. Thereasons are as follows.

The separator is in contact with the polymer electrolyte membrane andfunctions as an electrode; hence, the contact resistance is preferablylow, and to this end it is necessary to make the top of the passage (theconvexity) flat and ensure the contact area as large as possible.

Since the separator needs to have a function of supplying reaction gasthrough the passage uniformly, it is necessary to make the standing wallof the rectangular cross section of the passage vertical and ensure thearea of the passage as large as possible. Furthermore, since the fuelcell is a stacked structure, it is necessary to make the standing wallof the passage as vertical as possible and ensure the compressivestrength of the separator, and thus configure a stacked structure thatis hard to buckle.

However, as described above, it is technically difficult to formconcavities and convexities (passages) having the preferred rectangularcross section mentioned above using a concave-convex mold whilepreventing a crack associated with a decrease in the thickness of themetal thin plate and suppressing a “warp” due to elasticity recoveryafter the press-molding.

Thus, in view of the present circumstances of the conventionaltechnologies, an issue of the present invention is to, whenpress-molding a metal thin plate to produce a fuel cell separator usinga mold having a molding surface in which a concavity and convexity areadjacent to each other, form concavities and convexities (passages)having an acute angle-like cross section in which the top of theconcavities and convexities is flat and the standing wall shared betweenthe concavity and convexity is vertical while preventing a crackassociated with a decrease in the thickness of the metal thin plate andsuppressing a “warp” due to elasticity recovery after the press-molding;and an object of the present invention is to provide a mold that solvesthe issue, a method for producing a fuel cell separator using the moldas the upper and lower molds, and a fuel cell separator.

Solution to Problem

The present inventors conducted extensive studies on the method to solvethe issue mentioned above. As a result, the present inventors have foundthat concavities and convexities (passages) having an acute angle-likerectangular cross section in which the top of the convexity is flat andthe standing wall shared between the concavity and convexity is verticalcan be formed on the metal thin plate while a crack associated with adecrease in the thickness of the metal thin plate is prevented and a“warp” due to elasticity recovery after the press-molding is suppressedwhen a metal thin plate is press-molded using a mold that has a moldingsurface in which a concavity and convexity are adjacent to each otherand in which an arc-like minute concave surface is formed on the uppersurface of the concavities and convexities and an arc-like minute convexsurface is formed on the lower surface of the concavities andconvexities.

The present invention has been made based on the finding mentionedabove, and the gist of the present invention is as follows.

[1]

A fine mold for molding a fuel cell separator by press-molding a metalthin plate for a fuel cell separator to produce the fuel cell separator,comprising:

a molding surface in which a concavity and convexity are adjacent toeach other with a prescribed period, and

(i) an arc-like minute concave surface on an upper surface of theconcavity and convexity, and

(ii) an arc-like minute convex surface on a lower surface of theconcavity and convexity.

[2]

The fine mold for molding a fuel cell separator according to [1],wherein a depth D of the arc-like minute concave surface satisfiesFormula (1) below:

0.1·R<D<R  (1)

R: a curvature radius of a shoulder connecting the upper surface or thelower surface of the concavity and convexity and a vertical surface.

[3]

The fine mold for molding a fuel cell separator according to [1] or [2],wherein a height H of the arc-like minute convex surface satisfiesFormula (2) below:

0.1·R<H<R  (2)

R: a curvature radius of a shoulder connecting the upper surface or thelower surface of the concavity and convexity and a vertical surface.

[4]

The fine mold for molding a fuel cell separator according to [2] or [3],wherein the R satisfies Formula (3) below:

R(mm)=α·t  (3)

α: a constant

t: a thickness of the metal thin plate for a fuel cell separator.

[5]

The fine mold for molding a fuel cell separator according to any one of[1] to [4], wherein the fine mold for molding a fuel cell separator is apress mold.

[6]

The fine mold for molding a fuel cell separator according to any one of[1] to [4], wherein the fine mold for molding a fuel cell separator isof a roll type.

[7]

A method for producing a fuel cell separator by press-molding a metalthin plate for a fuel cell separator to produce a fuel cell separator,the method comprising:

(i) performing preliminary molding by press-molding the metal thin plateso that a cross section of the metal thin plate becomes a wavelike formwith a prescribed period; and subsequently(ii) press-molding the metal thin plate having a wavelike cross sectionwith the prescribed period using, as each of upper and lower molds, thefine mold for molding a fuel cell separator according to any one of [1]to [6] having a molding surface in which a concavity and convexity areadjacent to each other with the same period as the prescribed period.[8] The method for producing a fuel cell separator according to [7],wherein the upper and lower molds are each a press mold.[9]

The method for producing a fuel cell separator according to [7], whereinthe upper and lower molds are each of a roll type.

[10]

A fuel cell separator produced by the method for producing a fuel cellseparator according to any one of [7] to [9], wherein the fuel cellseparator has an acute angle-like cross section of a gas passage anddoes not have a warp.

[11]

The fuel cell separator according to [10], wherein the warp provides awarp index Z defined by Formula (4) below of less than or equal to 3.0

a warp index Z=(Hs/L)×100  (4)

Hs: a height of the warp (mm)

L: a length of the separator (mm).

Advantageous Effects of Invention

According to the present invention, there can be provided a fuel cellseparator that does not have a “warp,” has a high compressive strength,has a small contact resistance with a polymer electrolyte membrane, andcan supply reaction gas (hydrogen and air) uniformly.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing an embodiment of a molding surface of a finemold for molding a fuel cell separator.

FIG. 2 is a diagram showing change of a cross section of a metal thinplate in a process of producing a fuel cell separator. (a) shows awavelike cross section with a period of p after press-molding(preliminary molding), and (b) shows a concave-convex cross section witha period of p of the metal thin plate for a fuel cell separator.

FIG. 3 is a diagram showing cross sections of a metal thin plate for afuel cell separator of the present invention and a conventional metalthin plate for a fuel cell separator. FIG. 3A shows a concave-convexcross section formed by press-molding a metal thin plate having awavelike cross section with a period of p=1.5 mm using, as the upper andlower molds, molds each having a molding surface that has an arc-likeminute concave surface on the upper surface of concavities andconvexities and has an arc-like minute convex surface on the lowersurface of the concavities and convexities,

and FIG. 3B shows a concave-convex cross section formed by press-moldinga metal thin plate having a wavelike cross section with a period ofp=1.5 mm using, as the upper and lower molds, molds each having aconventional molding surface that does not have an arc-like minuteconcave surface on the upper surface of concavities and convexities anddoes not have an arc-like minute convex surface on the lower surface ofthe concavities and convexities either.

FIG. 4 is a diagram showing technical meaning of a warp index.

FIG. 5 is a diagram showing concave-convex cross sections of separatorsproduced as examples; FIG. 5A shows a comparative example, and FIG. 5Bshows an example of the present invention.

DESCRIPTION OF EMBODIMENTS

A fine mold for molding a fuel cell separator of the present invention(hereinafter may be referred to as “the present invention mold”) is amold that press-molds a metal thin plate for a fuel cell separator toproduce a fuel cell separator, and comprises a molding surface in whicha concavity and convexity are adjacent to each other, and

(i) an arc-like minute concave surface on the upper surface of theconcavity and convexity, and(ii) an arc-like minute convex surface on the lower surface of theconcavity and convexity.

A method for producing a fuel cell separator of the present invention(hereinafter may be referred to as “the present invention productionmethod”) is a method that press-molds a metal thin plate for a fuel cellseparator to produce a fuel cell separator, and comprises

(i) press-molding the metal thin plate so that the cross section of themetal thin plate becomes a wavelike form with a prescribed period(preliminary molding), and subsequently(ii) press-molding the metal thin plate having a cross section in awavelike form with the prescribed period using, as the upper and lowermolds, the present invention molds each having a molding surface inwhich a concavity and convexity are adjacent to each other with the sameperiod as the prescribed period.

A fuel cell separator of the present invention (hereinafter may bereferred to as “the present invention separator”) is a fuel cellseparator produced by the present invention production method.

First, the present invention mold is described based on the drawing.

In FIG. 1, an embodiment of the molding surface of a fine mold formolding a fuel cell separator (the present invention mold) is shown. Asshown in FIG. 1, on the molding surface of the mold, concavities andconvexities that form concave-convex passages on a metal thin plate fora fuel cell separator (hereinafter may simply be referred to as “metalthin plate”) are formed adjacent to each other with a prescribed period.That is, on the molding surface of the mold, concavities and convexitiesare formed with a prescribed period in such a manner that an uppersurface 1 and a lower surface 2 of the concavities and convexities arecontinuously adjacent to each other to a vertical surface 3 via ashoulder 4.

An arc-like minute concave surface 1 a is formed on the upper surface 1of the concavities and convexities of the molding surface, and similarlyan arc-like minute convex surface 2 a is formed on the lower surface 2of the concavities and convexities of the processing surface. This is astructural feature of the present invention mold.

The metal thin plate is not particularly limited to a metal thin platefor a special fuel cell separator, but is preferably a titanium thinplate or an austenite-based stainless steel thin plate, for example.

The present inventors have found that, when a metal thin plate that ispress-molded to obtain a wavelike cross section in advance ispress-processed using molds each having the molding surface shown inFIG. 1 as the upper and lower molds, concavities and convexities(passages) having an acute angle-like rectangular cross section in whichthe top of the convexity is flat and the standing wall shared betweenthe concavity and convexity is vertical can be formed on the metal thinplate while a crack associated with a decrease in the thickness of themetal thin plate is prevented and a “warp” due to elasticity recoveryafter the press-molding is suppressed. This is a finding that forms thebasis of the present invention mold.

The present inventors surmise that the reason why the present inventionmold can form concavities and convexities (passages) having an acuteangle-like rectangular cross section while preventing a crack associatedwith a decrease in the thickness of the metal thin plate and suppressinga “warp” due to elasticity recovery after the press-molding is asfollows.

During press-molding, the arc-like minute concave surface formed on theupper surface of the concavities and convexities of the molding surfaceand the arc-like minute convex surface formed on the lower surface ofthe concavities and convexities of the processing surface will act sothat the plastic flow of the metal thin plate is directed to thestanding wall side (the shoulder) and will thus complete plasticdeformation. As a result, the upper and lower surfaces will be providedwith a uniform compressive strain, and deformation due to elasticityrecovery after the press-molding will be suppressed to the utmost; thus,passages having an acute angle-like rectangular cross section can beformed.

In the fine mold having the molding surface shown in FIG. 1 (the presentinvention mold), the depth D of the arc-like minute concave surfaceformed on the upper surface of the concavities and convexities of theprocessing surface preferably satisfies Formula (1) below.

0.1·R<D<R  (1)

R: the curvature radius of the shoulder connecting the upper surface orthe lower surface of the concavities and convexities and the verticalsurface

If the depth D of the arc-like minute concave surface is less than orequal to “0.1·R”, the plastic flow of the metal thin plate does notoccur toward the standing wall side (the shoulder), and the top cannotbe made flat sufficiently; thus, the depth D is set to more than“0.1·R.” The depth D is more preferably more than or equal to “0.2·R”.

On the other hand, if the depth D is more than or equal to “R”, thethickness of the upper surface of the passage is made non-uniform, or acrack occurs due to excessive squashing; thus, the depth D is set toless than “R.” The depth D is more preferably less than or equal to“0.5·R”.

In the fine mold having the processing surface shown in FIG. 1 (thepresent invention mold), the height H of the arc-like minute convexsurface formed on the lower surface of the concavities and convexitiesof the processing surface preferably satisfies Formula (2) below.

0.1·R<H<R  (2)

R: the curvature radius of the shoulder connecting the upper surface orthe lower surface of the concavities and convexities and the verticalsurface

The reason why the height H of the arc-like minute convex surface isprescribed in the range mentioned above is the same as the reason whythe depth D of the arc-like minute concave surface is prescribed in therange mentioned above.

A more preferred range is, as in the case of the depth D of the arc-likeminute concave surface, 0.2·R<H<0.5·R, and also the reason forprescribing the range in this range is the same.

The present inventors have found experimentally that R in Formula (1)and Formula (2) above (the curvature radius of the shoulder connectingthe upper surface or the lower surface of the concavities andconvexities and the vertical surface) has an appropriate value for thereasons of avoiding a crack of the shoulder and forming a verticalstanding wall, and has an optimum range shown by Formula (3) below inrelation to the thickness of the metal thin plate.

R(mm)=α·t  (3)

α: a constant

t: the thickness (mm) of the metal thin plate for a fuel cell separator

α is a constant that is determined experimentally, and is 0.5 to 1.5.

Since the thickness of the metal thin plate is usually 50 to 200 μm, R(mm) is preferably selected in the range of (0.5 to 1.5)×(0.05 to 0.2)(mm). If α is less than 0.5, a crack of the shoulder is likely to occur;and if α exceeds 1.5, the roundness of the shoulder is made large, andit becomes difficult to obtain a vertical standing wall. α preferablysatisfies α≧0.7 and α≦1.3.

The present invention mold may be either of a press mold and a rollmold.

Next, a method for producing a fuel cell separator using the fine moldsfor molding a fuel cell separator as the upper and lower molds isdescribed.

A method for producing a fuel cell separator of the present invention(hereinafter may be referred to as “the present invention productionmethod”) is a method that press-molds a metal thin plate for a fuel cellseparator to produce a fuel cell separator, and comprises

(i) press-molding the metal thin plate so that the cross section of themetal thin plate becomes a wavelike form with a prescribed period(preliminary molding), and subsequently(ii) press-molding the metal thin plate having a cross section in awavelike form with the prescribed period using, as the upper and lowermolds, the present invention molds each having a molding surface inwhich a concavity and convexity are adjacent to each other with the sameperiod as the prescribed period.

FIG. 2 shows the change of a cross section of a metal thin plate in aprocess of producing a fuel cell separator. In FIG. 2(a), a wavelikecross section with a period of P after press-molding (preliminarymolding) is shown; and in FIG. 2(b), a concave-convex cross section witha period of P of the metal thin plate for a fuel cell separator isshown.

Using a mold having a wavelike molding surface with a prescribed periodof P, a metal thin plate for a fuel cell separator is press-molded so asto have the cross section shown in FIG. 2(a) (preliminary molding).Subsequently, the metal thin plate having a wavelike cross section witha period of P is press-molded using, as the upper and lower molds, thepresent invention molds each having a molding surface in which aconcavity and convexity are adjacent to each other with the period of P.The cross section of the press-molded metal thin plate is shown in FIG.2(b).

Here, in FIG. 3, cross sections of a metal thin plate for a fuel cellseparator of the present invention and a conventional metal thin platefor a fuel cell separator are shown. FIG. 3A shows a concave-convexcross section with a height h of 0.6 mm that is formed by press-moldinga metal thin plate having a wavelike cross section with a period ofp=1.5 mm using, as the upper and lower molds, molds each having amolding surface that has an arc-like minute concave surface on the uppersurface of the concavities and convexities and has an arc-like minuteconvex surface on the lower surface of the concavities and convexities(the present invention mold).

FIG. 3B shows a concave-convex cross section with a height h of 0.6 mmthat is formed by press-molding a metal thin plate having a wavelikecross section with a period of p=1.5 mm using, as the upper and lowermolds, conventional molds each having a molding surface that does nothave an arc-like minute concave surface on the upper surface of theconcavities and convexities and does not have an arc-like minute convexsurface on the lower surface of the concavities and convexities either.

When the concave-convex cross section shown in FIG. 3A and theconcave-convex cross section shown in FIG. 3B are compared, it can beseen that an upper surface 5 b of the concave-convex cross section shownin FIG. 3B has “roundness” and the concave-convex cross section isgenerally not in an acute angle-like form, whereas an upper surface 5 aof the concave-convex cross section shown in FIG. 3A is “flat” and theconcave-convex cross section is generally in an acute angle-like form.

It is surmised that the reason why the upper surface of theconcave-convex cross section formed using the present invention molds asthe upper and lower molds is “flat” and the concave-convex cross sectionis in an acute angle-like form is that, as described above, duringpress-molding, the arc-like minute concave surface formed on the uppersurface of the concavities and convexities of the molding surface andthe arc-like minute convex surface formed on the lower surface of theconcavities and convexities of the processing surface in the presentinvention mold act so that the plastic flow of the metal thin plate isdirected toward the standing wall side (the shoulder) and plasticdeformation is thus completed; consequently, the upper and lowersurfaces are provided with a uniform compressive strain, and deformationdue to elasticity recovery after the press-molding is suppressed to theutmost; thus, an acute angle-like concave-convex cross section isformed.

Furthermore, it is surmised that, since an acute angle-likeconcave-convex cross section is formed on the entire metal thin plate bythe action of the arc-like minute concave surface formed on the uppersurface of the concavities and convexities of the molding surface andthe arc-like minute convex surface formed on the lower surface of theconcavities and convexities of the processing surface, a “warp” due toelasticity recovery after the press-molding is suppressed.

Next, the present invention separator is described. In the presentinvention separator, since the cross section of the gas passages is anacute angle-like concave-convex cross section, there is generally no“warp,” the compressive strength is high, the contact resistance withthe polymer electrolyte membrane is small, and reaction gas (hydrogenand air) can be supplied uniformly.

The present inventors have introduced a warp index defined by Formula(4) below in order to evaluate the “warp” of the present inventionseparator.

The warp index Z=(Hs/L)×100  (4)

Hs: the height of the warp (mm)

L: the length of the separator (mm)

In FIG. 4, the technical meaning of the warp index is shown. As shown inFIG. 4, in a separator in which the length L of one side is warped, themaximum distance of the convex surface of the concave-convex passagefrom the surface that the four ends of the separator form (shown by theline connecting both ends in the figure) is defined as the height of thewarp Hs. As is clear from FIG. 4, the warp index Z is preferably assmall as possible.

Examples

Next, Examples of the present invention are described; the conditions inExamples are only examples of the conditions used to verify thefeasibility and effect of the present invention, and the presentinvention is not limited to the examples of the conditions. The presentinvention may use various conditions to the extent that they do notdepart from the spirit of the present invention and that the object ofthe present invention is achieved.

Examples

An austenite-based stainless steel foil with a thickness of 100 μm onwhich a wavelike cross section with a period of 1.5 mm was formed byordinary press-molding was press-molded using the molds shown in Table 1as the upper and lower molds, and thereby concave-convex passages with aheight of 0.6 mm were formed; thus, a fuel cell separator was produced.The size of the fuel cell separator was set to 250 mm×150 mm, and thesize of the concave-convex passage portion was set to 100 mm×200 mm.

TABLE 1 Concavities and convexities of molding surface, period p: 1.5 mmDepth D of arc-like Height H of arc-like minute curved minute curvedCurvature radius Thickness of surface of upper surface of lower R ofshoulder metal thin plate t Mold surface surface (mm) (μm) Notes A 0.030.03 0.10 100 Example B 0.05 0.05 0.15 150 Example C 0 0 0.10 100Conventional Example D 0 0 0.15 150 Conventional Example

A cross section of the concave-convex passages of the fuel cellseparator was observed visually, and the “warp” was evaluated by thewarp index; a sample with Z of less than or equal to 3.0% was evaluatedas good, and a sample with Z of more than 3.0% was evaluated as poor.The results are shown in Table 2. The size in the longitudinal directionof the separator of the examples is 250 mm; when the warp index is lessthan or equal to 3.0%, the height of the warp is less than or equal to7.5 mm. When the height of the warp is less than or equal to 7.5 mm,separators can be assembled without problems when they are stacked usinghigh tensile bolts and terminal plates having sufficient rigidity.

TABLE 2 Upper and Concave-convex cross section Warp Thickness lowerUpper Lower Standing index Separator μm molds surface surface wall WholeZ Notes 1 100 A Flat Flat Vertical Acute Good Example angle- like 2 150B Flat Flat Vertical Acute Good Example angle- like 3 100 C Round RoundOblique Not Poor Conventional acute Example angle- like 4 150 D RoundRound Oblique Not Poor Conventional acute Example angle- like

From Table 2, it can be seen that, in Examples, fuel cell separatorseach having an acute angle-like concave-convex cross section and nothaving a “warp” have been obtained. The degree of flatness of the upperand lower surfaces and the degree of verticality of the standing wall inthe concave-convex cross section were evaluated quantitatively by, asshown in FIG. 5, drawing a center line in the plate thickness in theconcave-convex cross section of the separator and using the shape of thecenter line. For the degree of flatness, the lengths of the flatportions L_(F) of the upper and lower surfaces were compared; and theL_(F) of Examples was a length of approximately 2.5 times the L_(F) ofConventional Examples, and provided a good flat shape. For the height ofthe standing wall, the angles θ of the standing wall portions werecompared; and the θ of Examples was lower than the θ of ConventionalExamples by approximately 4 degrees, and provided a good standing wallshape.

Thus, the present invention separator is a fuel cell separator that hasa small contact resistance with the polymer electrolyte membrane and cansupply reaction gas uniformly.

INDUSTRIAL APPLICABILITY

As described above, the present invention can provide a fuel cellseparator that does not have a “warp,” has a high compressive strength,has a small contact resistance with a polymer electrolyte membrane, andcan supply reaction gas uniformly. When the fuel cell separator is used,the efficiency of the fuel cell is improved; thus, the present inventionhas high applicability in battery manufacturing industries.

REFERENCE SIGNS LIST

-   1 upper surface of concavities and convexities-   1 a arc-like minute concave surface-   2 lower surface of concavities and convexities-   2 a arc-like minute convex surface-   3 vertical surface-   4 shoulder-   5 a, 5 b upper surface of convexity-   d depth of arc-like minute concave surface-   h height of arc-like minute convex surface-   R curvature radius of shoulder-   Hs height of warp-   L length of separator

1. A fine mold for molding a fuel cell separator by press-molding ametal thin plate for a fuel cell separator to produce the fuel cellseparator, comprising: a molding surface in which a concavity andconvexity are adjacent to each other with a prescribed period, and (i)an arc-like minute concave surface on an upper surface of the concavityand convexity, and (ii) an arc-like minute convex surface on a lowersurface of the concavity and convexity.
 2. The fine mold for molding afuel cell separator according to claim 1, wherein a depth D of thearc-like minute concave surface satisfies Formula (1) below:0.1·R<D<R  (1) R: a curvature radius of a shoulder connecting the uppersurface or the lower surface of the concavity and convexity and avertical surface.
 3. The fine mold for molding a fuel cell separatoraccording to claim 1, wherein a height H of the arc-like minute convexsurface satisfies Formula (2) below:0.1·R<H<R  (2) R: a curvature radius of a shoulder connecting the uppersurface or the lower surface of the concavity and convexity and avertical surface.
 4. The fine mold for molding a fuel cell separatoraccording to claim 2, wherein the R satisfies Formula (3) below:R(mm)=α·t  (3) α: a constant t: a thickness of the metal thin plate fora fuel cell separator.
 5. The fine mold for molding a fuel cellseparator according to claim 1, wherein the fine mold for molding a fuelcell separator is a press mold.
 6. The fine mold for molding a fuel cellseparator according to claim 1, wherein the fine mold for molding a fuelcell separator is of a roll type.
 7. A method for producing a fuel cellseparator by press-molding a metal thin plate for a fuel cell separatorto produce a fuel cell separator, the method comprising: (i) performingpreliminary molding by press-molding the metal thin plate so that across section of the metal thin plate becomes a wavelike form with aprescribed period; and subsequently (ii) press-molding the metal thinplate having a wavelike cross section with the prescribed period using,as each of upper and lower molds, the fine mold for molding a fuel cellseparator according to any one of claims 1 to 6 having a molding surfacein which a concavity and convexity are adjacent to each other with thesame period as the prescribed period.
 8. The method for producing a fuelcell separator according to claim 7, wherein the upper and lower moldsare each a press mold.
 9. The method for producing a fuel cell separatoraccording to claim 7, wherein the upper and lower molds are each of aroll type.
 10. A fuel cell separator produced by the method forproducing a fuel cell separator according to claim 7, wherein the fuelcell separator has an acute angle-like cross section of a gas passageand does not have a warp.
 11. The fuel cell separator according to claim10, wherein the warp provides a warp index Z defined by Formula (4)below of less than or equal to 3.0a warp index Z=(Hs/L)×100 Hs: a height of the warp (mm) L: a length ofthe separator (mm).